1. Field of the Invention
The present invention provides a receiver of a code division multiple access communication system, and more particularly, a receiver capable of enhancing receiving efficiency.
2. Description of the Prior Art
In the scope of wireless communication technologies, code division multiple access (CDMA) technology is a spread-spectrum communication technology, which has advantages, such as anti-interference, multipath rejection, multiple access, etc., to enhance signal reliability and secrecy, and can provide a channel for multiple users. The CDMA technology uses pseudo-random (PN) codes to broaden a bandwidth of modulated signals, so as to hide the modulated signals in the environment as noise. Therefore, the CDMA technology can protect transmitted signals from wiretapping, and is utilized broadly in wireless communication systems, such as the 3rd generation mobile communication system, the global positioning system, etc.
In a CDMA communication system, a server device, or transmitting end, radios to user equipment using different base stations. Obviously, distances between the user equipment and the base stations are not same, and signals between the user equipment and the base stations may be interfered with by landforms, surface features, etc., which makes the user equipment suffer from a near-far problem. The near-far problem means that among signals outputted from the base stations, signals having lower power are interfered with by signals having higher power, causing the user equipment to be unable to lock the signals having lower power. In a 3G communications system, the prior art outputs a power control command for controlling signal power of each base station through a physical layer protocol, so that signals received by the user equipment from different base stations have the same power toward the user equipment, and therefore the near-far problem can be solved. However, a GPS radios positioning signals from satellites using a one-way ranging method, so a GPS receiver cannot control signal power of the satellites, and suffers the near-far problem, and cause the receiver to not be able to locate positions and navigate correctly.
The GPS detects positions according to radio waves and time differences between transmitters and a receiver, and the receiver can provide exact information of latitude, altitude, speed, and time based on ranging codes from the transmitter. The ranging codes are C/A(coarse/acquisition), and P code(precision codes). The C/A codes are provided for general users, and provide lower precision of positioning than the P codes. The P codes are constructed by shorter chips, and provided for military uses, which will not be mentioned further. In the GPS, each satellite has a unique C/A code for acquisition and, most important, navigation data. In the satellite, the navigation data undergoes a convolution operation with the C/A code, and is modulated with a carrier. The navigation data includes ephemeris data, almanac data, launch time of the navigation data, timing calibration data, status of all satellites, propagation delay parameters of ionosphere, UTC parameters, etc.
The GPS receiver calculates positions according to a triangular positioning theorem. When the receiver receives signals from single satellite, the receiver can calculate a position data with a microprocessor to find out how far and where is the satellite. When there are two available satellites, the receiver can calculate a circle range formed by intersection of two sphere signals provided by the satellites. When there are three available satellites, two intersection points are formed in three sphere signals provided by the satellites, and one is on the earth, and the other is in space beyond the atmosphere, so that the receiver can find out longitude and latitude. Furthermore, when there are four available satellites, the receiver can find out longitude, latitude, and altitude. Therefore, the receiver requires four satellite signals for correct positioning.
If one satellite signal received by the receiver has power higher than other satellite signals received by the receiver, the receiver suffers the near-far problem. As a result, the receiver can only lock on the satellite signal having maximum power. Since the GPS receiver requires four satellite signals, the receiver cannot work properly when suffering from the near-far problem. For example, when a user navigates using a vehicle GPS receiver in downtown, signals from satellites may be interfered with by cloud layers, the windshield of the car, or buildings, which result in effects of propagation loss, multipath propagation, shadowing, fading, delay spread, etc., and lead to the near-far problem.